From cosmic deceleration to acceleration: new constraints from SN Ia and BAO/CMB

We use type Ia supernovae (SN Ia) data in combination with recent baryonic acoustic oscillations (BAO) and cosmic microwave background (CMB) observations to constrain a kink-like parametrization of the deceleration parameter (q). This q-parametrization can be written in terms of the initial (q_i) and present (q₀) values of the deceleration parameter, the redshift of the cosmic transition from deceleration to acceleration (z_t) and the redshift width of such transition (τ). By assuming a flat space geometry, q_i = 1/2 and adopting a likelihood approach to deal with the SN Ia data we obtain, at the 68% confidence level (C.L.), that: z_t = 0.56^+0.13_−0.10, τ = 0.47^+0.16_−0.20 and q₀ = −0.31^+0.11_−0.11 when we combine BAO/CMB observations with SN Ia data processed with the MLCS2k2 light-curve fitter. When in this combination we use the SALT2 fitter we get instead, at the same C.L.: z_t = 0.64^+0.13_−0.07, τ = 0.36^+0.11_−0.17 and q₀ = −0.53^+0.17_−0.13. Our results indicate, with a quite general and model independent approach, that MLCS2k2 favors Dvali-Gabadadze-Porrati-like cosmological models, while SALT2 favors ΛCDM-like ones. Progress in determining the transition redshift and/or the present value of the deceleration parameter depends crucially on solving the issue of the difference obtained when using these two light-curve fitters.